Biology

New "giant dragon" dinosaur offers clues to T-rex's weirdly tiny arms

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An artist's impression of Meraxes gigas, a new species of giant predatory dinosaur discovered in Patagonia
Jorge A Gonzalez
An artist's impression of Meraxes gigas, a new species of giant predatory dinosaur discovered in Patagonia
Jorge A Gonzalez
Meraxes gigas had disproportionately tiny arms for its size, like many carnivorous dinosaurs
Carlos Papolio

Paleontologists have described a new species of giant carnivorous dinosaur from a fairly complete skeleton in Patagonia. The discovery could help shed new light on the mystery of why such powerful predators had almost comically tiny arms.

The new species has been named Meraxes gigas, with the first part referencing a dragon from George R. R. Martin's Song of Ice and Fire novels and the second part coming from the Greek word for “giant.” And giant it was – the dinosaur weighed over 4 tons, stood almost 11 m (36 ft) long and had a huge skull that measured 1.3 m (4.2 ft) long. That head was adorned with a series of crests, bumps, furrows and hornlets, giving it a fearsome appearance.

But one thing about this new species wasn’t quite so giant – its arms. Like many carnivorous theropods, its forelimbs were disproportionately tiny, measuring less than half the length of its femur. That means Meraxes couldn’t reach its mouth, making it unlikely the arms were used for anything related to feeding. But if that’s the case, what did it use them for?

That’s a question that paleontologists have pondered for decades, with many hypothesizing that they’re vestigial limbs – evolutionary leftovers that shrink over time as the animals no longer need them. But the newly discovered skeleton has some features that suggest otherwise.

Meraxes gigas had disproportionately tiny arms for its size, like many carnivorous dinosaurs
Carlos Papolio

“The fossil of M. gigas shows never seen before, complete regions of the skeleton, like the arms and legs that helped us to understand some evolutionary trends and the anatomy of Carcharodontosaurids – the group that M. gigas belongs to,” said Juan Canale, project lead on the study. “I’m convinced that those proportionally tiny arms had some sort of function. The skeleton shows large muscle insertions and fully developed pectoral girdles, so the arm had strong muscles.”

The researchers propose a few other possible uses for these arms, like helping the huge animal get up off the ground after a rest, or to hold onto its partner while mating. That’s in line with suggestions from other scientists in the past, but without being able to observe behavior, we may never know the answer for sure.

Whatever that function was, it must have been important, because the new study shows that tiny forelimbs evolved multiple times in at least three distinct lineages – Carcharodontosaurids like Meraxes, abelisaurids like Carnotaurus, and tyrannosaurids like … well, Tyrannosaurus. And these aren’t direct ancestors of each other either. Meraxes lived about 95 million years ago, becoming extinct some 20 million years before T-rex evolved.

We may not have any definitive answers, but at least we have a scary new villain for any future Jurassic World movies.

The research was published in the journal Current Biology.

Source: CONICET via Scimex

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5 comments
anthony88
Is it possible that they hopped like kangaroos?
DJ's "Feed Me Doggie"
Take me in your arms, rock me, rock me a little while. Oh little arm darling, rock me, rock me a little while.
I'm sorry darling. but your arms are too short to do anything but get you out of bed!
I'ma gonna go find m a Quatropus. Four log, lovely arms, and nowhere to put a wedding ring.
Expanded Viewpoint
Anthony, you would see an elephant hop like a Kangaroo long before you ever saw a T-Rex do it! Their weight was much too great to allow that kind of movement. In British Columbia, T-Rex track ways were found, and they placed one foot in front of the other, unlike modern avian or human tracks. But they did lift their feet backwards as they were raised, like birds of today do. We pull our feet forward as we walk and make dual sets of prints. T-Rex must have looked quite odd while walking, and it's no wonder they must have plodded along slowly, they may have tripped and fallen over if they moved too fast!
jsopr
"Consider this fossil of a strange, apparently bipedal ape. Its forelegs appear withered and weak compared to all other apes, altogether unsuited to brachiation, or even moving along the ground like its cousin the gorilla. What could the purpose of such small, unmuscled limbs have been? Sexual display? Helping to upright itself after a fall from its precarious posture? We may never know..."
styly1
Yes they did hop like birds and kangaroos do. These creatures also walk when not at pace ie hunting or running away. The tiny powerful forelimbs are claspers for feeding / mating. If they were not anchored to prey this process would be akin to bobbing for apples is for us for a bipedal animal. You cannot dismiss the structure of an animal due to assumptions. The mere fact that this design has been spontaneously repeated by evolution for one thing. Harken to when "experts" thought dinosaurs dragged their tails all day . An assumption i can only explain from what they saw in their toys in the sand box box rather than an acutual living creature.